Walkthrough: Multithreading with the BackgroundWorker Component (C# and Visual Basic)

This walkthrough demonstrates how to create a multithreaded application that searches a text file for occurrences of a word. It demonstrates:

To create the user interface

  1. Open a new Visual Basic or C# Windows Application project, and create a form named Form1.

  2. Add two buttons and four text boxes to Form1.

  3. Name the objects as shown in the following table.

    Object

    Property

    Setting

    First button

    Name , Text

    Start, Start

    Second button

    Name , Text

    Cancel, Cancel

    First text box

    Name , Text

    SourceFile, ""

    Second text box

    Name , Text

    CompareString, ""

    Third text box

    Name , Text

    WordsCounted, "0"

    Fourth text box

    Name , Text

    LinesCounted, "0"

  4. Add a label next to each text box. Set the Text property for each label as shown in the following table.

    Object

    Property

    Setting

    First label

    Text

    Source File

    Second label

    Text

    Compare String

    Third label

    Text

    Matching Words

    Fourth label

    Text

    Lines Counted

To create a BackgroundWorker component and subscribe to its events

  1. Add a BackgroundWorker component from the Components section of the ToolBox to the form. It will appear in the form's component tray.

  2. Set the following properties for the BackgroundWorker1 object in Visual Basic or the backgroundWorker1 object in C#.

    Property

    Setting

    WorkerReportsProgress

    True

    WorkerSupportsCancellation

    True

  3. In C# only, subscribe to the events of the backgroundWorker1 object. At the top of the Properties window, click the Events icon. Double-click the RunWorkerCompleted event to create an event handler method. Do the same for the ProgressChanged and DoWork events.

To define the method that will run on a separate thread

  1. From the Project menu, choose Add Class to add a class to the project. The Add New Item dialog box is displayed.

  2. Select Class from the templates window and type Words.vb or Words.cs in the name field.

  3. Click Add. The Words class is displayed.

  4. Add the following code to the Words class:

    Public Class Words
        ' Object to store the current state, for passing to the caller. 
        Public Class CurrentState
            Public LinesCounted As Integer 
            Public WordsMatched As Integer 
        End Class 
    
        Public SourceFile As String 
        Public CompareString As String 
        Private WordCount As Integer = 0
        Private LinesCounted As Integer = 0
    
        Public Sub CountWords(
            ByVal worker As System.ComponentModel.BackgroundWorker,
            ByVal e As System.ComponentModel.DoWorkEventArgs
        )
            ' Initialize the variables. 
            Dim state As New CurrentState
            Dim line = "" 
            Dim elapsedTime = 20
            Dim lastReportDateTime = Now
    
            If CompareString Is Nothing OrElse
               CompareString = System.String.Empty Then 
    
               Throw New Exception("CompareString not specified.")
            End If 
    
            Using myStream As New System.IO.StreamReader(SourceFile)
    
                ' Process lines while there are lines remaining in the file. 
                Do While Not myStream.EndOfStream
                    If worker.CancellationPending Then
                        e.Cancel = True 
                        Exit Do 
                    Else
                        line = myStream.ReadLine
                        WordCount += CountInString(line, CompareString)
                        LinesCounted += 1
    
                        ' Raise an event so the form can monitor progress. 
                        If Now > lastReportDateTime.AddMilliseconds(elapsedTime) Then
                            state.LinesCounted = LinesCounted
                            state.WordsMatched = WordCount
                            worker.ReportProgress(0, state)
                            lastReportDateTime = Now
                        End If 
    
                        ' Uncomment for testing. 
                        'System.Threading.Thread.Sleep(5) 
                    End If 
                Loop 
    
                ' Report the final count values.
                state.LinesCounted = LinesCounted
                state.WordsMatched = WordCount
                worker.ReportProgress(0, state)
            End Using 
        End Sub 
    
        Private Function CountInString(
            ByVal SourceString As String,
            ByVal CompareString As String
        ) As Integer 
            ' This function counts the number of times 
            ' a word is found in a line. 
            If SourceString Is Nothing Then 
                Return 0
            End If 
    
            Dim EscapedCompareString =
                System.Text.RegularExpressions.Regex.Escape(CompareString)
    
            ' To count all occurrences of the string, even within words, remove 
            ' both instances of "\b". 
            Dim regex As New System.Text.RegularExpressions.Regex(
                "\b" + EscapedCompareString + "\b",
                System.Text.RegularExpressions.RegexOptions.IgnoreCase)
    
            Dim matches As System.Text.RegularExpressions.MatchCollection
            matches = regex.Matches(SourceString)
            Return matches.Count
        End Function 
    End Class
    

To handle events from the thread

  • Add the following event handlers to your main form:

    Private Sub BackgroundWorker1_RunWorkerCompleted( 
        ByVal sender As Object, 
        ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs
      ) Handles BackgroundWorker1.RunWorkerCompleted
    
        ' This event handler is called when the background thread finishes. 
        ' This method runs on the main thread. 
        If e.Error IsNot Nothing Then
            MessageBox.Show("Error: " & e.Error.Message)
        ElseIf e.Cancelled Then
            MessageBox.Show("Word counting canceled.")
        Else
            MessageBox.Show("Finished counting words.")
        End If 
    End Sub 
    
    Private Sub BackgroundWorker1_ProgressChanged( 
        ByVal sender As Object, 
        ByVal e As System.ComponentModel.ProgressChangedEventArgs
      ) Handles BackgroundWorker1.ProgressChanged
    
        ' This method runs on the main thread. 
        Dim state As Words.CurrentState = 
            CType(e.UserState, Words.CurrentState)
        Me.LinesCounted.Text = state.LinesCounted.ToString
        Me.WordsCounted.Text = state.WordsMatched.ToString
    End Sub
    

To start and call a new thread that runs the WordCount method

  1. Add the following procedures to your program:

    Private Sub BackgroundWorker1_DoWork( 
        ByVal sender As Object, 
        ByVal e As System.ComponentModel.DoWorkEventArgs
      ) Handles BackgroundWorker1.DoWork
    
        ' This event handler is where the actual work is done. 
        ' This method runs on the background thread. 
    
        ' Get the BackgroundWorker object that raised this event. 
        Dim worker As System.ComponentModel.BackgroundWorker
        worker = CType(sender, System.ComponentModel.BackgroundWorker)
    
        ' Get the Words object and call the main method. 
        Dim WC As Words = CType(e.Argument, Words)
        WC.CountWords(worker, e)
    End Sub 
    
    Sub StartThread()
        ' This method runs on the main thread. 
        Me.WordsCounted.Text = "0" 
    
        ' Initialize the object that the background worker calls. 
        Dim WC As New Words
        WC.CompareString = Me.CompareString.Text
        WC.SourceFile = Me.SourceFile.Text
    
        ' Start the asynchronous operation.
        BackgroundWorker1.RunWorkerAsync(WC)
    End Sub
    
  2. Call the StartThread method from the Start button on your form:

    Private Sub Start_Click() Handles Start.Click
        StartThread()
    End Sub
    

To implement a Cancel button that stops the thread

  • Call the StopThread procedure from the Click event handler for the Cancel button.

    Private Sub Cancel_Click() Handles Cancel.Click
        ' Cancel the asynchronous operation. 
        Me.BackgroundWorker1.CancelAsync()
    End Sub
    

You can now test the application to make sure it works correctly.

To test the application

  1. Press F5 to run the application.

  2. When the form is displayed, enter the file path for the file you want to test in the sourceFile box. For example, assuming your test file is named Test.txt, enter C:\Test.txt.

  3. In the second text box, enter a word or phrase for the application to search for in the text file.

  4. Click the Start button. The LinesCounted button should begin incrementing immediately. The application displays the message "Finished Counting" when it is done.

To test the Cancel button

  1. Press F5 to start the application, and enter the file name and search word as described in the previous procedure. Make sure that the file you choose is large enough to ensure you will have time to cancel the procedure before it is finished.

  2. Click the Start button to start the application.

  3. Click the Cancel button. The application should stop counting immediately.

This application contains some basic error handling. It detects blank search strings. You can make this program more robust by handling other errors, such as exceeding the maximum number of words or lines that can be counted.

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